1. Technical Field
The present invention relates to a robot having a function for measuring the mass of a workpiece that is held by a robot having a force measurement unit.
2. Description of Related Arts
In order to inspect defective parts and products that are being manufactured, the mass of a workpiece is measured using a robot having a force measurement unit. To be more specific, a robot hand having a force measurement unit in a robot wrist part holds a workpiece. Then, the mass of the workpiece is measured from an output value of the force measurement unit then.
Such mass measurement is carried out, for example, by checking whether or not a plurality of workpieces that are being carried, when in fact only one workpiece needs to be held. Alternately, such mass measurement is also carried out to determine whether a workpiece is good or bad, based on whether or not the mass of a single workpiece is in a predetermined range.
Japanese Unexamined Patent Publication No. 6-31667 discloses conducting good product inspection, parts mounting inspection and type inspection of a workpiece, based on a mass measurement value, so as to sort workpieces. Furthermore, Japanese Unexamined Patent Publication No. 6-31667 discloses, in order to measure the mass of a workpiece, programming robot operations such that the mass of the workpiece applies to a force measurement unit vertically, and finding the difference between the output of the force measurement unit then and a predetermined reference mass.
Furthermore, Japanese Unexamined Patent Publication No. 5-57667 discloses a configuration which provides a pair of force measurement units (load cells) above and below a robot hand, respectively. Then, the mass of a workpiece is measured from the outputs of the measurement units when the workpiece is held from above and from below. In Japanese Unexamined Patent Publication No. 5-57667, in a state in which a hand holds a workpiece and a pair of load sensors are provided in a vertical direction, the workpiece is held still, and the mass of the workpiece is determined by subtracting the weight value of the upper load cell from the weight value of the lower load cell.
Incidentally, FIG. 7A and FIG. 7B are side views of a robot according to related art. The robot illustrated in these drawings is a vertical articulated robot, and a robot hand 3 is attached to a tip part 2 of an arm of the robot. Then, a force sensor 5 is provided between the tip part 2 and the hand 3 of the arm.
First, as illustrated in FIG. 7A, a robot 1 is made still in a state in which a workpiece W is not held, and the output of the force sensor 5 is recorded. Then, as illustrated in FIG. 7B, the hand 3 only holds the workpiece W (without changing the position and posture of the robot 1), and, in a state of being still, the output of the force sensor 5 is recorded. Then, the mass of the workpiece W is determined based on the deviation of the two outputs of the force sensor 5.
In this connection, in the event the force sensor 5 is a load cell with one axis, as illustrated in FIG. 7A and FIG. 7B, upon measurement of the force sensor 5, the force sensor 5 is required to be directed vertically downward. However, in the event the force sensor 5 is a force sensor of three or more axes, if the position and posture are the same when the workpiece W is not held and when the workpiece is held, the force sensor 5 may be in another posture than being directed vertically downward.
As described earlier, when measuring the mass of a workpiece W, the robot 1 needs to stay still. The reason is that, while the robot 1 is operating, not only the gravitational forces of the hand 3 and workpiece W, but also inertial forces such as the forces produced by acceleration/deceleration of the robot 1, the centrifugal force, the Coriolis force, and so on, are detected by the force sensor 5. Then, if the robot 1 is stopped still such that these gravitational forces and inertial forces do not apply, the robot 1 requires time to decelerate, time to stop, and time to re-accelerate. Consequently, the cycle time of the robot 1 increases significantly.
Furthermore, according to Japanese Unexamined Patent Publication No. 6-31667 and Japanese Unexamined Patent Publication No. 5-57667, upon measurement of a workpiece, the posture of the robot 1 needs to be changed such that the weight of the workpiece applies to the force measurement unit vertically.
The present invention has been made in view of the above backgrounds, and it is therefore an object of the present invention to provide a robot having a workpiece mass measurement function, whereby the mass of a workpiece can be acquired accurately even during handling operations.